1. Field of the Invention
The field of the invention is wireless communications. The present invention pertains to inter-frequency handover and inter-frequency measurement reporting.
2. Related Art and Other Considerations
In a typical cellular radio system, a geographical area is divided into cell areas served by base stations which are connected to a radio network. Each user (mobile subscriber) in the cellular radio system is provided with a portable, pocket, hand-held, or car-mounted mobile station (user equipment unit or UE) which communicates voice and/or data with the mobile radio network. Each base station includes a plurality of channel units including a transmitter, a receiver, and a controller and may be equipped with an omnidirectional antenna for transmitting equally in all directions or with directional antennas, each directional antenna serving a particular sector cell. Each user equipment unit (UE) also includes a transmitter, a receiver, a controller, and a user interface and is identified by a specific user equipment unit (UE) identifier.
In a cellular radio communications system, a handover operation allows an established radio connection to continue when a mobile radio participating in that connection moves between cells in the system. Handover is typically initiated when the signal strength or signal quality of the radio connection with an origination base station falls below a predetermined threshold value. Often, a low signal strength or a poor signal quality indication means that the user equipment unit (UE) is near a border between the two cells. If the user equipment unit (UE) moves closer to a destination cell or to a clearer line of unobstructed sight, handover of the radio connection to the destination cell usually results in improved radio transmission and reception.
In some cellular systems, a handover operation requires physically breaking the connection with the origination cell and then reestablishing the connection with the destination cell, i.e., a “break-before-make” switching operation. Such hard handover techniques are typically employed in Time Division Multiple Access (TDMA) and Frequency Division Multiple Access (FDMA) type cellular systems.
On the other hand, “soft” handover techniques may be employed in Code Division Multiple Access (CDMA) type cellular systems. CDMA is an increasingly popular type of access for cellular communications because a higher spectrum efficiency is achieved compared to FDMA and TDMA techniques which means that more cellular users and/or services can be supported. In addition, a common frequency band allows simultaneous communication between a user equipment unit (UE) and plural base stations. Signals occupying the common frequency band are discriminated at the receiving station through spread spectrum CDMA waveform properties based on the use of a high speed, pseudo-noise (PN) code. These high speed PN codes are used to modulate signals transmitted from the base stations and the user equipment units (UEs). Transmitter stations using different PN codes (or a PN code offset in time) produce signals that can be separately demodulated at a receiving station. The high speed PN modulation also allows the receiving station to advantageously generate a received signal from a single transmitting station by combining several distinct propagation paths of the transmitted signal.
In CDMA, therefore, a user equipment unit (UE) need not switch frequency when handoff of a connection is made from one cell to another. As a result, a destination cell can support a connection to a user equipment unit (UE) at the same time the origination cell continues to service the connection. Since the user equipment unit (UE) is always communicating through at least one cell during handover, there is no disruption to the call. Hence, the term “soft handover.” In contrast to hard handover, soft handover is a “make-before-break” switching operation.
Deciding which cells to involve in handover often requires coordination between the user equipment unit (UE) and the radio network. For example, in Wideband CDMA (WCDMA), the user equipment unit (UE) maintains a list of cells to monitor for potential handover purposes. The list of cells maintained by the user equipment unit (UE) includes cells comprising an “active list”, as well as adjacent cells which (although not in the active list) are to be monitored. The user equipment unit (UE) continuously updates its list of cells to monitor based on information communicated to the user equipment unit (UE) by the network. For example, the network may provide, via a message such as a measurement control message, an initial list of cells via a message such as a measurement control message. The initial list of cells may be, for example, a list of cells which neighbor the cell in which the user equipment unit (UE) is located. Thereafter the network may update the user equipment unit (UE) regarding what cells should be included in the active set using a message such as an active set update message.
The user equipment unit (UE) monitors, e.g., makes measurements regarding, the base station control or broadcast channel of each of the cells included in the list maintained by the user equipment unit (UE). The monitoring results (e.g., measurements) are transmitted to the network where, based on this monitoring, the network makes a determination as to what cells should be in the active set (e.g., what cells should be added, replaced, or removed).
As indicated above, in CDMA a user equipment unit (UE) need not necessarily switch frequency when handoff of a connection is made from one cell to another. Moreover, at any given moment, the user equipment unit (UE) may be in radio contact over the same radio frequency with plural base stations, i.e., the “active set” of base stations/cells for the user equipment unit (UE). This type of soft hand-over described above (pertaining to the use of the same frequency) is also known as a intra-frequency soft handover or active set update procedure.
In order to know how to perform an intra-frequency soft handover, measurements must be performed relative to certain control channels transmitted, e.g., from the base stations in the active set and from adjacent cells. The properties of a general control channel used for measurements are that the control channel have fixed power and be monitorable by the user equipment unit (UE) over the anticipated coverage area of the cell, allowing an extra margin for delays in user equipment unit (UE) detection and reporting. The measurements can be, for example, certain signal strength or signal to noise ratio (e.g., signal to interference ratio) measurements for the control channels. The values of the measurements of the control channels from the base stations are compared, with the results of the comparison being utilized to determine how the handover is to be performed (e.g., which cells to include or exclude from the active set).
In the soft handover context, the control channels transmitted from the base stations in the active set and adjacent cells, and which are measured for purposes of the handover, are distinguishable from one another despite being on the same frequency. In one particular CDMA context, the control channel utilized for the handover measurements is a physical control channel known as a common pilot channel (CPICH), formerly referred to as a primary common control physical channel (P CCPCH). Typically, in connection with the different cells for handover, the user equipment unit (UE) measures the CPICH of the cells that should be monitored (e.g., those for the base stations in the active set). The CPICH can be regarded as the equivalent to what is commonly called the pilot, or perch, channel for other systems.
Thus, as alluded to above, employment of measurements (e.g., of the CPICH) for the purpose of handover typically involves some type of measurement reporting, e.g., a reporting of measurements from the user equipment unit (UE), for example. Topics such as reporting criteria and reporting events for such measurements, the maintenance of an active set of base stations supported by event driven measurement reporting from the user equipment unit (UE), as well as handover in general, are described e.g., in one or more of the following (all of which are incorporated herein by reference in their entirety):                (1) U.S. Pat. No. 6,445,917, entitled “Mobile Station Measurements With Event-Based Reporting”;        (2) U.S. patent application Ser. No. 09/344,122, filed Jun. 24, 1999, entitled “Network-Evaluated Handover Assisted By Both Mobile and Base-Stations”;        (3) U.S. Pat. No. 6.490.461, entitled “Power Control Based On Combined Quality Estimates”;        (4) U.S. Pat. No. 6.438.375, entitled “Coordinating Different Types of Messages Sent To Mobile Radios In A Mobile Communications System”.In addition, background information may be provided by one or more of the following United States patents, all of which are incorporated herein by reference: U.S. Pat. No. 5,594,718; U.S. Pat. No. 5,697,055; U.S. Pat. No. 5,267,261; U.S. Pat. No. 5,848,063.        
While intra-frequency soft handover is an advantageous feature of CDMA, there are occasions when a user equipment unit (UE) needs to switch to a new frequency. The changing or switching of a connection involving a user equipment unit (UE) from one frequency to another is known as an inter-frequency handover. Due to limitations of the user equipment unit (UE), a “soft” inter-frequency handover is essentially impossible, or at the very best very difficult, to achieve. Therefore, the inter-frequency handover typically must be a hard handover. Such being the case, it is highly desirable that the telecommunications network allocate the appropriate cells for the active set on the new frequency as quickly as possible. In order to do so, however, the network needs relevant measurement information on the new frequency from the user equipment unit (UE). But obtaining the relevant measurement information on the new frequency from the user equipment unit (UE) after the inter-frequency handover is executed is time-consuming, and works against the objective of allocating the active set on the new frequency as quickly as possible.
What is needed, therefore, and an object of the present invention, is a technique, for facilitating rapid inter-frequency handover.